1. Field of the Invention
This invention relates to circuits for generating microprocessor clocks, and particularly to circuits for generating clocks using a voltage-controlled oscillator.
2. Description of Background
High-frequency voltage-controlled oscillators (VCOs) are extremely important for applications such as processor clock generation and/or distribution, wired and/or wireless communication, system synchronization, and frequency synthesis. Research on VCOs for the past decade has been concentrated in the areas of high frequency, lower jitter, lower operating voltage and power, and increasing the frequency tuning range. Many of these design goals are achieved only at the expense of some or all of the other performance objectives.
Variability of oscillator frequency with respect to temperature, as well as process variation and voltage changes, can have a significant effect on performance, cost, and yield. Complementary metal-oxide semiconductor (CMOS) ring oscillators are commonly used for microprocessor clock generation and high speed input/output (I/O). These oscillators typically have a fequency sensitivity to temperature of about 0.1% to 0.2%/degree C. Operation in applications that require extremely wide temperature ranges, such as mil-spec (military specification) of −55 Celsius (C) to +125C may not be feasible. Accordingly, the state of the art may be advanced by a voltage controlled oscillator that overcomes these drawbacks.